Cardiac cell excitation is crucially controlled by several large superfamilies of ion channels that regulate the depolarization and repolarization phases, trigger calcium release in individual myocytes, and govern the conduction and coordinated contractile function of the heart. For example, voltage-gated outward K+ currents control the shape and duration of the action potential (AP) in cardiac myocytes. Classically, the time-dependent K+-channels are divided into the transient outward current (Ito) that underlies the first phase (phase 1) of AP repolarization and the ultra-rapid, rapid, and slow delayed rectifier currents (IKur, IKr, and IKs) that determine the middle and late phases of repolarization.
Long QT syndrome (LQTS) is a disorder characterized by a prolonged myocardial repolarization, an abnormally long QT interval in surface (ECG) and recurrent episodes of ventricular tachyarrhythmias, which may lead to syncope, cardiac arrest, or sudden death. LQTS may be congenital or induced, but both forms of the syndrome are caused by defects in the ion channel mechanism controlling cardiac cell excitation. Congenital LQTS is caused by mutations of cardiac ion channel genes; 7 chromosomal loci and 7 specific genes have been identified to date, and is subdivided into two different syndromes, Romano-Ward syndrome and Jervell and Lang-Nielsen (JLN) syndrome. Romano-Ward syndrome is characterized by familial occurrence with autosomal dominant inheritance, QT prolongation, and ventricular tachyarrhythmias, while JLN syndrome is characterized by familial occurrence with autosomal recessive inheritance, congenital deafness, QT prolongation, and ventricular arrhythmias. Based on genetic background, 6 types of LQTS and 2 types of JLN syndrome have been identified, as shown in FIGS. 1-2. The prevalence of LQTS is difficult to estimate, but, based on the currently increasing frequency of diagnosis, LQTS is expected to occur in 1 in 1,000-2,000 individuals. LQTS is an underdiagnosed disorder, especially because at least 10-15% of LQTS gene carriers have normal QTc duration.
The most prevalent congenital forms of LQTS, LQT1 and LQT2 and LQT3 are related to loss of function of delayed rectifier currents IKs or IKr due to mutations in the α-subunit KvLQT1 (KCNQ1) or HERG (KCNH2), respectively gain of function mutations of the voltage-gated cardiac sodium channel α-subunit SCN5A. Linkage studies have revealed that null mutations in HERG (KCNH2), a gene located on chromosome 7 that encodes the pore-forming subunit of a voltage-gated potassium channel IKr, the LQT2 form of LQTS. Gain-of-function mutations in the putative inactivation domain of the human cardiac Na+ channel, SCN5A result in LQT3. Mutations in KCNE2, a minK-related peptide (MiRP1) that may co-assemble with HERG to form IKr, may cause LQT6. JLN syndrome, a variant of LQTS, is associated with prolonged repolarization, deafness, and sudden death. The disease is related to mutations in both of the alleles encoding KvLQT1. Loss of function mutations in the inward rectifier potassium channel Kir2.1 (KCNJ2) cause LQT7.
Torsade de pointes (TdP) is an atypical polymorphic ventricular tachycardia most often associated with QT prolongation in both congenital and acquired forms of LQTS. Several experimental and clinical observations using monophasic action potential suggest a significant role for early afterdepolarization (EAD)-induced triggered activity in the genesis of TdP. These EADs can induce reentry and TdP if there is dispersion of repolarization across the wall of the heart. EADs may also be induced either by interventions that decrease the repolarizing K+ currents (e.g., class III antiarrhythmic drugs) or increase the inward currents INa or ICa.
However, the mechanisms behind the various types of LQTS and associated disorders are still not fully understood. Mice models have been instrumental in understanding the assembly and role of ionic channels in regulating repolarization in the heart. However, because the mouse heart is small, the resting heart rate is 600 beats/min, and the APD is very short, different time-dependent currents play a role in repolarization as compared with those in humans. A need exists, therefore, for an animal model system that is more similar to humans in terms of its repolarizing currents.
While mice and rats remain the most widely used animal models in genetic investigation of many human diseases, there remain certain diseases for which mice and rats are not an accurate model for human disease. The ionic mechanisms of repolarization in adult rats and mice differ from larger species, including humans (the primary ion currents controlling repolarization in adult rats and mice is Ito); therefore, use of these species is not considered appropriate for modeling cardiac ion channel disorders such as LQTS. To date there are no genetic models to study the long-term effects of suppression of IKr and IKs in a model in which these channels control cardiac repolarization and the shape of its action potential.
Other non-murine laboratory animal species that may be more suitable for in vivo electrophysiology and other studies to explore human cardiac diseases and study potential therapies include dog, monkey, swine, rabbit, ferret, and guinea pig. For example, the rabbit's larger size and slower heart rate allow performance of electrophysiologic studies that more accurately model human physiology. The rabbit heart is also more similar to the human heart in terms of the contractile proteins that are expressed and the ion channels important for repolarization. For example, similar to the case in humans, the calcium-insensitive Ito plays an important role in the rabbit cardiomyocyte; it is likely coded by Kv1.4, Kv4.2 and Kv4.3, while in humans Ito is coded by primarily Kv4.3. IKs is also easily detected in the rabbit heart, but is less prominent than in guinea pigs and humans. This current is unique in its slow activation kinetics—it occurs on the order of seconds—and is constant among different species. However, its deactivation is slow in guinea pig, but relatively fast in the rabbit and human. Thus, IKs is believed to be particularly important in situations in which repolarization is impaired by either inward-current enhancement or outward-current reduction (like β-adrenergic drive and impaired rapid delayed-rectifier function). Indeed, under resting conditions, IKs seems to be smaller, but it is activated under sympathetic stimulation. Similar to humans, IKr plays a major role in the rabbit heart. As a result, the rabbit is particularly sensitive to class III drugs and responds with prolongation of the QT interval and EADs. The dominant role of IKr compared with IKs is likely due to lower steady-state levels of minK transcript. Of note, Tsuji et al. demonstrated recently in rabbits that complete AV block leads to substantial QT prolongation and high incidence of spontaneous TdP. Both IKr and IKs were downregulated by about 50% in this model.